Aerospace applications require more robust and more reliable motor drive systems with high power density and high operation efficiency (more compact in size and weight). These requirements are even more critical for the newly developing “more electrical aircraft”, for its much higher electrical power ratings and much larger number of AC motor drive systems on board. A conventional AC motor drive system utilizes an inverter (e.g., a voltage source inverter) under control of a PWM (pulse width modulation) generator and switch driver to convert DC power to multi-phase AC power for supply to the motor. In a current control AC drive system, flux and torque producing current components (represented by direct-axis and quadrature-axis vectors Id and Iq, respectively) are controlled to achieve desired motor speed and torque over the entire speed range of the AC motor. Various conventional current control structures for a voltage source inverter-fed motor drive require sufficient voltage to maintain suitable current regulation over the entire operating speed range of the drive. This condition is not likely to be met in aerospace applications, because a voltage limitation will typically be exerted on the system to achieve maximum power density and efficiency. With such voltage constraints, not only will the system operating point drift off from the desired state, but the system may encounter stability problems when using a vector control method. The voltage limitation can also come from abnormally low DC bus or DC bus disturbances introduced by system dynamics, such as power interruptions. The system will typically experience other unexpected transient situations, e.g., caused by load dynamics or by noise, errors and spikes due to inverter, controller and motor non-idealities.
A conventional current controller uses only the speed regulator and start/stop processes as references. The conventional current controller shuts down the inverter gating (e.g., implemented using insulated gate bipolar transistors; “IGBTs”) whenever any defined threshold is reached. To resume normal operation, the system typically must go through a start-up process when normal operating condition recovers, because the system has lost the control during the shutdown period. For a motor drive system using a sensorless control algorithm to determine rotor position/speed, this process may have to wait until the motor fully stops to be able to re-start. Another way to mitigate this issue is to increase the design margin. This means a trade-off with over-all system size and weight, however, which is undesirable.